1. Field of the Invention
The present invention relates to an improved method of cleaning a nuclear steam generator or the other tube bundle heat exchanger by removing the buildup of sedimentation or "sludge" which accumulates in the bottom of a heat exchanger vessel through utilization of a repetitive shock wave induced in the sludge and in flushing fluid. The shock wave serves to effectively and safely loosen the products of corrosion and other elements which settle at the bottom of the heat exchanger or steam generator and thereby facilitates their easy removal through flushing and vacuuming the vessel.
2. Description of the Prior Art
One of the major components in a power generating facility such as a nuclear power plant is the steam generator or heat exchanger portion of the facility. Large scale heat exchanger systems are essentially comprised of a primary system which contains a large number of individual tubes which have fluid circulating through them and a secondary system which consists of a second fluid surrounding said tubes contained within a housing which enwraps both systems. Heat is transferred from the fluid running through these heat exchanger tubes to the fluid in the secondary system which is itself eventually turned to steam. The steam, in turn, generates power.
These heat exchangers or steam generators have experienced many problems due to the buildup of products of corrosion, oxidation, sedimentation and comparable chemical reactions within the heat exchanger. The problem of magnetite buildup at the junctions of the primary heat exchanger tubes and the support plates for those tubes, and further magnetite buildup within the crevices between the tubes and their support plates was extensively treated in U.S. Pat. No. 4,320,528. That patent addressed the use of ultrasonic methods to facilitate the removal of the magnetite from those junctions. The present inventors are the same as the inventors in U.S. Pat. No. 4,320,528.
At the bottom of the heat exchanger vessel is a tube sheet. This is thick metal plate which acts as the support base for numerous heat exchanger tubes. In addition to the problems of magnetite buildup at the junctions and inside the crevices of the primary heat exchanger tubes and their support plates, a second problem has also troubled steam generators for many years. There is a buildup of sedimentation or "sludge" which accumulates in the bottom of heat exchanger vessels. This sludge includes copper oxide, magnetite and other oxidation or corrosion products which have not adhered to the tubing or other surfaces and therefore accumulate at the bottom. The sludge pile rests on top of the tube sheet and on top of the higher elevation support plates and may form a thick layer. The sludge further accumulates in the crevices between the tube sheet and the primary heat exchanger tubes which are embedded in the tube sheet for support and also accumulates on the tube support plates. The problem of removing the sludge which enters the deep crevices in the tube sheet was addressed in presently pending patent application Ser. No. 06/370,826 filed on 4/22/82. Patent application Ser. No. 06/370,826 solves the problem of removing sludge from the deep crevices through use of specialized ultrasonic waves which are directed in a certain way to produce the desired result.
In addition to the above two prior art references, the following prior art patents address the problem of cleaning a nuclear steam generator or else keeping it clean before it becomes dirty through the use of ultrasonics:
1. U.S. Pat. No. 2,664,274 issued to Worn et al. PA0 2. U.S. Pat. No. 2,987,086 issued to Branson. PA0 3. U.S. Pat. No. 3,033,710 issued to Hightower et atl. PA0 4. U.S. Pat. No. 3,240,063 issued to Sasaki et al. PA0 5. U.S. Pat. No. 3,295,596 issued to Ostrofsky et al. PA0 6. U.S. Pat. No. 3,433,669 issued to Kouril. PA0 7. U.S. Pat. No. 3,428,811 issued to Harriman et al. PA0 8. U.S. Pat. No. 3,447,965 issued to Teumac et al. PA0 9. U.S. Pat. No. 3,854,996 issued to Frost et al. PA0 10. U.S. Pat. No. 4,120,699 issued to Kennedy et al. PA0 11. U.S. Pat. No. 4,167,424 issued to Jubenville et al. PA0 1. Chemical Cleaning of BWR and Steam Water system at Dresden Nuc. Pw. Station, Obrecht et al., pp 1-18, (10/26/60) 21st Ann. Conf. of Eng. PA0 2. Special Tech. Pub. 42 (1962) ASTM Role of Cavitation in Sonic Energy Cleaning, by Bulat. PA0 3. R & D Status Report Nuclear Power Division, which appeared on pages 52 through 54 of the April 1981 issue of the EPRI Journal. The Article was by John J. Taylor.
All of the above referenced patents have been extensively discussed in both U.S. Pat. No. 4,320,528 or else in presently pending patent application Ser. No. 06/370,826 filed on 4/22/82. The following two prior art publications have also been discussed in these references:
The buildup of sludge on the tube sheet and upper tube support plates degrades the heat transfer process from the fluid in the primary system to the fluid in the secondary system, and may also restrict secondary fluid flow as well as producing a stagnant zone which enhances corrosion of the tubes, tube sheet and support plates. As addressed in patent application Ser. No. 06/370,826, the sludge which enters crevices within the tube sheet creates further problems and serves to damage the heat exchanger tubes. As a result, it is very important to clean the heat exchanger or steam generator to effectively remove the sludge from the surface of the tube sheet. All of the prior art discussed above employs the use of ultrasonics. While the methods discussed in the prior art, especially those in U.S. Pat. No. 4,320,528 and application No. 06/370,826, are very effective and valuable, the requirement of using ultrasonics has several significant disadvantages. First, in order to generate the ultrasonic waves, expensive transducers must be used. This requires considerable effort and expense to bring the ultrasonic transducers to the site of the steam generator and then putting them in their proper place at the location of the steam generator. Second, in order to achieve an effective level of ultrasonic waves, it is often necessary to cut away a portion of the steam generator wall and put the face of the transducer at the location of the cut away portion. Many owners of the power plant which incorporates a steam generator are very reluctant to have a portion of the wall cut away and then later welded back in place after the steam generator has been cleaned.
A third problem which arises with prior art applications is the use of corrosive chemicals to assist in the cleaning operation. While the chemicals serve to clean and remove the sludge, they also serve to eat away at the various components of the steam generator. Therefore, it is desirable to find a method of cleaning which does not require the use of corrosive chemicals. One method known in the prior art is called water lancing. This is in effect the use of a jet of water which is shot into the sludge pile for the purpose of loosening the sludge. There are some problems with this water lancing process. The loosening process is not very effective because it is difficult to penetrate to the interior of the tube bundle and in addition there may be a problem of using the jet of water to impinge against the heat exchanger tubes at that location. The jet of water might cause sludge particles to reflect onto and then off the heat exchanger tubes, thereby possibly resulting in damage to these tubes.
Therefore, although the use of ultrasonics combined with chemicals and the use of a jet of water are all known in the prior art for cleaning and removing sludge at the bottom of a heat exchanger or steam generator, none of these methods can be employed without the significant problems discussed above. At present, there has been no prior art method for effectively removing the sludge through very quick, inexpensive method which does not require the use of chemicals or the cutting away of a portion of the steam generator.